Progress in methods to detect and treat cancer has led to increased five-year survival rates. However, cancer mortality rates have not decreased proportionately (Welch, et al. (2000) JAMA 283:2975-2978). Some transformed cells can survive medical treatment and lay dormant for years before they emerge to cause relapse in a patient. Nontransformed cells can force tumor cells to assume a normal morphology and phenotype by a process called “Contact Normalization” (Glick & Yuspa (2005) Semin. Cancer Biol. 15:75-83; Rubin (2003) Adv. Cancer Res. 90:1-62).
Increased anchorage-independent growth and migration distinguish cancer cells from their nontransformed precursors (Frisch & Screaton (2001) Curr. Opin. Cell Biol. 13:555-562; Giancotti & Buoslahti (1999) Science 285:1028-1032). Src kinase, a membrane-bound tyrosine kinase, phosphorylates Cas (Crk associated substrate) to promote these fundamental hallmarks of tumor cell growth (Brabek, et al. (2004) Oncogene 23:7406-7415; Cho & Klemke (2000) J. Cell Biol. 149:223-236; Goldberg, et al. (2003) J. Biol. Chem. 278:46533-46540; Honda, et al. (1998) Nat. Genet. 19:361-365; Huang, et al. (2002) J. Biol. Chem. 277:27265-27272; Klemke, et al. (1998) J. Cell Biol. 140:961-972; Shin, et al. (2004) J. Biol. Chem. 279:38331-38337).
Src has been implicated in many human cancers (Frame (2002) Biochim. Biophys. Acta 1602:114-130; Irby & Yeatman (2000) Oncogene 19:5636-5642). Increased Src activity has been reported in cancers of the colon (Cartwright, et al. (1994) J. Clin. Invest 93:509-515; Cartwright, et al. (1989) J. Clin. Invest 83:2025-2033; Cartwright, et al. (1990) Proc. Natl. Acad. Sci. USA 87:558-562), and particularly in metastatic colon cancer that invades liver (Termuhlen, et al. (1993) J. Surg. Res. 54:293-298). Increased Src activity is also found in cancers of the pancreas (Lutz, et al. (1998) Biochem. Biophys. Res. Commun. 243:503-508), lung (Mazurenko, et al. (1992) Eur. J. Cancer 28:372-377); neural cells (Bjelfman, et al. (1990) Mol. Cell. Biol. 10:361-370; Bolen, et al. (1985) Proc. Natl. Acad. Sci. USA 82:7275-7279), muscle (Rosen, et al. (1986) J. Biol. Chem. 261:13754-13759), ovaries (Budde, et al. (1994) Cancer Biochem. Biophys. 14:171-175; Wiener, et al. (1999) Clin. Cancer Res. 5:2164-2170), esophagus (Kumble, et al. (1997) Gastroenterology 112:348-356), skin (Bjorge, et al. (1996) Biochem. Cell Biol. 74:477-484; Munshi, et al. (2000) J. Immunol. 164:1169-1174), stomach (Takeshima, et al. (1991) Jpn. J. Cancer Res. 82:1428-1435), and breast (Ottenhoff-Kalff, et al. (1992) Cancer Res. 52:4773-4778; Rosen, et al. (1986) supra; Verbeek, et al. (1996) J. Pathol. 180:383-388). Indeed, metastatic cell growth can be reduced by agents that inhibit Src activity (Lutz, et al. (1998) supra; Roginskaya, et al. (1999) Leukemia 13:855-861; Staley, et al. (1997) Cell Growth Differ. 8:269-274).
Cas is an important component of the focal adhesion complex signaling network (Bouton, et al. (2001) Oncogene 20:6448-6458) that also includes FAK, Grb2, Shc, and paxillin (Schlaepfer, et al. (1999) Prog. Biophys. Mol. Biol. 71:435-478; Sieg, et al. (1999) J. Cell Sci. 112 (Pt 16):2677-2691). After phosphorylation by Src, Cas can bind to other proteins including Crk, PI-3-kinase, Nc, and PLCγ (Burnham, et al. (1996) Oncogene 12:2467-2472; Sakai, et al. (1994) EMBO J. 13:3748-3756; Vuori, et al. (1996) Mol. Cell. Biol. 16:2606-2613). Src transformation of homozygous null Cas knockout (CasKO) cells does not fully promote their anchorage-independence or ability to migrate. These transformed growth characteristics can be conferred to CasKO cells by transfection with wild-type Cas (Brabek, et al. (2005) Mol. Cancer. Res. 3:307-315; Goldberg, et al. (2003) supra; Honda, et al. (1998) supra; Huang, et al. (2002) supra).
Src phosphosphorylates Cas to inhibit Fhl1 expression thereby promoting nonanchored cell growth and migration (Shen, et al. (2006) Cancer Res. 66:1543-1552). Fhl1 is composed of four and half LIM domains and can move between intercellular junctions (Huang, et al. (2002) Yi. Chuan Xue. Bao. 29:953-958), focal adhesions, and the nucleus (Brown, et al. (1999) J. Biol. Chem. 274:27083-27091) to affect gene expression (Qin, et al. (2005) FEBS Lett. 579:1220-1226; Taniguchi, et al. (1998) Mol. Cell. Biol. 18:644-654). Transfection studies demonstrate that Fhl1 specifically blocks nonanchored tumor cell growth and migration, but does not affect “normal” anchored cell growth (Shen, et al. (2006) supra). Therefore, Fhl1 acts as a true tumor suppressor rather than as a general mitotic inhibitor. Analysis of human clinical specimens suggests that Fhl1 expression is suppressed in some human tumors including those of breast, kidney, and prostate. Furthermore, Fhl1 expression is suppressed by Src, and is restored during Contact Normalization of transformed cells (Shen, et al. (2006) supra). Thus, Fhl1 acts downstream of Src and Cas to suppress the anchorage-independence and motility that leads to metastatic cell growth.